Carolyn McCosh says she has loved "real books, the printed-on-paper kind," ever since she got her St. Louis Public Library card in first grade. The 45-year-old had little interest in e-books - until Christmas.

Along with millions of others, she unwrapped a Kindle Fire, a gift from her boyfriend and "a huge defining moment for me." Since then, she has bought $100 worth of e-books - from histories to mysteries - and wonders, "Am I a traitor to printed books?"

McCosh is part of an unprecedented surge in e-book sales that's changing publishing and challenging traditional bookstores.

It's reflected on USA TODAY's Best-Selling Books list, which tracks combined sales of e-book and print editions. The latest list, based on sales data from Dec. 26 to Jan. 1, shows a remarkable burst of digital book sales after e-readers were unwrapped as gifts - for 42 of the top 50 titles, the e-book editions were the most popular format. The previous high, in July, was 25 of the top 50.

For e-books, "the two weeks after Christmas is what the two weeks before Halloween is to pumpkins," says Michael Norris, an analyst with Simba Information, a market research firm. After the post-holiday surge, he predicts, e-books will increase in "short bursts and slow trickles" the rest of the year.

Norris estimates that one in five U.S. adults are reading e-books on a variety of devices, from dedicated e-readers to tablets (like the Kindle Fire) that can be used to download movies, music, video games and more.

Forrester Research estimates that Amazon has sold 5 million Kindle Fires, priced at $199 each, since the device was released Nov. 14. Archrival Barnes & Noble has sold an estimated 2 million Nook Tablets ($249), released Nov. 17.

Both devices are designed, in part, to compete with Apple's iPad (the latest versions are priced from $500 to $830), which sold about 40 million units last year.

But even as the sales of e-books doubled from 10% of the overall market to 20% in 2011, print books still account for about 80% of the market.

Great time for readers

Michael Cader, founder of Publishers Lunch, an influential digital newsletter, says it's premature to call e-books a "game-changer" for publishing: "Lots of things are changing, but the game is still a lot like it was."

But the question for publishers and booksellers, including more than 350 independent stores that sell e-books in partnership with Google, is how fast changes are coming.

Without being specific, Russ Grandinetti, a vice president at online retailer Amazon (which dominates the markets for both e-readers and e-books), says its print and digital sales are both up, but "digital is growing significantly faster."

Downplaying print-vs.-digital questions, he adds, "For anyone who cares about books, it's never been a better time to be a reader. The choices have never been greater - what to read, when to read it, and how to integrate books into your daily life."

Barnes & Noble hopes its "Nook Boutiques" in its stores (703, down from 717 a year ago) can help sell its digital products with giveaways and instructions. Last week, it reported that Nook Tablet sales "exceeded expectations" but that it "over-anticipated" demand for its $99 Nook Simple Touch.

The chain's holiday sales of print books rose 4%, "the first increase in five years," says James Iannone, president of digital products. In part, he attributes that to the collapse of Borders, which went out of business last year, closing 600 stores. But he also says, "A lot of people still love physical books."

That was evident this holiday season, retailers' make-or-break time. In the weeks before Christmas, the number of titles on USA TODAY's best-seller list where the e-book outsold the hardcover or paperback edition declined - from 12 in the top 50 in early November to three the week before Christmas. Although there are gift cards for e-books, most people seem to like to wrap and inscribe books they give.

Sharing is hard

Now, millions of digital converts are exploring their new reading devices:

Steve Woods, 60, a St. Louis-based software consultant, says his Nook Tablet "has got me back into reading on the road. I like to read three or four books at the same time - going back and forth." (His current reading includes two titles near the top of USA TODAY's list: Walter Isaacson's biography Steve Jobs and Suzanne Collins' novel The Hunger Games.)

One thing he doesn't like: "It's not as easy or straightforward to lend e-books as with physical books." (Limits and restrictions on e-book lending are aimed at preventing piracy.)

Kathleen Lattea, 50, a Baltimore therapist, credits her Kindle Touch for prompting her to read "more broadly and more often" - from free classics, such as The Complete Sherlock Holmes, to "unknown" novelists, such as Joel Goldman, whose Motion to Kill she bought for 99 cents.

One complaint: It's hard to share her Kindle with her 24-year-old daughter. "She wants to read the latest Hunger Games while I am reading The Help when, of course, we can sit in the same room and read free library books. We're frugal and not duplicating devices or book purchases."

Each has advantages:

Laurie Ousley, 52, a naturopathic doctor in Kokomo, Ind., is enjoying her new Nook Tablet but has yet to read a book on it: "I've been too busy figuring out how to load music and use it for Facebook and e-mail." She's planning to download an e-book from her local library (although most major publishers have begun to restrict library lending of new e-books) and is "looking forward to having my books with me whenever and wherever I want them."

Denis Davis, 32, a radio program director in Florence, S.C., says his Nook Tablet's built-in speaker and "Read to Me" feature renewed his interest in reading with his 3-year-old son. With books such as A Charlie Brown Christmas, its recorded narrator "reads the words," Davis says, as "one turns the page. It is pretty cool at bedtime."

Theresa Foster, 24, a librarian in Barnegat, N.J., loves shifting formats, from audiobooks while driving, to physical books while working or studying, to her new back-lit iPad in bed at night, when "my husband doesn't have to hear the rustle of pages while he is trying to fall asleep."

At the library, Foster says one-third of last week's reference questions were about downloading e-books. Most of her teen patrons don't have their own e-readers, but she sees growing interest in e-books, "only an app away for many of them who already have iPods and smartphones."

At the same time, she worries if print books could go the way of CDs. She notes that "not everyone can afford e-readers or even a computer."

Norris, the market research analyst, doubts if e-books will "hit a true tipping point like music did. Music content has always required an electronic interface, be it a gramophone or an iPod," which is not the case with books.

Among questions facing publishers is how much e-books will cannibalize print. Cader says overall print sales dropped about 9% in 2011.

Meanwhile, publishers are experimenting with multiple formats.

On Jan. 23, Reagan Arthur Books, a division of Little, Brown, will release three versions of George Pelecanos' latest novel, What It Was- as a 99-cent e-book, a $9.99 paperback and a $35 limited-edition slip-cased hardcover.

Editorial director Reagan Arthur calls it an experiment, using the e-book (whose price will increase to $4.99 after a month) to attract readers new to Pelecanos. "We're interested in seeing the effects of low-priced e-books," she says. "We don't want to kill off print sales. We want to grow the total audience."

A new obsession

In St. Louis, McCosh, an administrative assistant at a construction firm, recalls when "I believed my love of books and reading could only be satisfied by turning the pages of an actual book. ... I did not want to read off a computer screen."

But with her Kindle, "I can access so much more material. I no longer carry books with me. I can change the font. All my books are on (virtual) shelves, just like my personal library in the real world."

She has found free books and "books I never knew existed... Why did I wait so long?"

She asks, "Will I never buy another book from a bookstore? How will I spend the hours I used to spend at the library? Or the afternoons at the bookstore? Will this obsession fade with time?"